Nanoscale pore structure cathode for high power applications and material synthesis methods

ABSTRACT

A lithium iron phosphate electrochemically active material for use in an electrode and methods and systems related thereto are disclosed. In one example, a lithium iron phosphate electrochemically active material for use in an electrode is provided including, a dopant comprising vanadium and optionally a co-dopant comprising cobalt.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a U.S. National Phase of InternationalApplication No. PCT/US2016/036473 for “NANOSCALE PORE STRUCTURE CATHODEFOR HIGH POWER APPLICATIONS AND MATERIAL SYNTHESIS METHODS”, filed onJun. 8, 2016. International Application No. PCT/US2016/036473 claimspriority from U.S. Provisional Application Ser. No. 62/185,457 for “HIGHPOWER CATHODE MATERIAL SYNTHESIS AND ASSOCIATED METHODS FORELECTROCHEMICAL ENERGY STORAGE DEVICES”, filed on Jun. 26, 2015, andU.S. Provisional Application Ser. No. 62/294,888 for “NANOSCALE PORESTRUCTURE CATHODE FOR HIGH POWER APPLICATIONS AND MATERIAL SYNTHESISMETHODS”, filed Feb. 12, 2016. The entire contents of each of theabove-referenced applications are incorporated herein by reference forall purposes.

FIELD

This application relates to materials and methods for batteryelectrodes, materials used therein, and electrochemical cells using suchelectrodes and methods of manufacture, such as lithium ion batteries.

BACKGROUND AND SUMMARY

Lithium-ion (Li-ion) batteries are a type of rechargeable battery whichproduces energy from electrochemical reactions. In a typical lithium ionbattery, the cell may include a positive electrode, a negativeelectrode, an ionic electrolyte solution that supports the movement ofions back and forth between the two electrodes, and a porous separatorwhich allows ion movement between the electrodes and ensures that thetwo electrodes are electrically isolated.

Li-ion batteries' success in the consumer electronics market hasresulted in their use in the transportation industry for hybrid electricvehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and electricvehicles (EVs). While rechargeable lithium-ion batteries have foundmultiple applications in portable electronics, high charge and dischargerates are secondary design considerations. However, when considering theuse of rechargeable lithium-ion batteries in the transportationindustry, the ability to sustain high charge and discharge rates becomesimportant. Transportation industry applications, as well as the everincreasing demand for more powerful portable electronic devices, hasprompted the need for batteries that can consistently maintain largecharge and discharge current densities. Thus, electrode materials havingirregular surfaces resulting in high interfacial surface areas and shortcharacteristic diffusion lengths, either through a porous structure ornanoscale primary particle size, are expected to provide lithium-ionbatteries with high power densities. Safety is also becoming animportant factor in the design of new Li-ion batteries, especially fortransportation applications.

To address the safety concern associated with oxide based cathodematerials, lithium iron phosphate (LFP) is considered a good replacementcandidate as it is thermodynamically stable and does not release oxygenupon decomposition. This is especially true for low voltage starter,start-stop, and mild-hybrid battery applications. When considering LFPas the cathode material, the characteristics in terms of morphology,chemical composition, and particle size may be carefully controlled.Because different LFP precursor materials and different synthesis routesare employed by material suppliers, special attention may first be givento impurities and ensuring the correct composition. The incorrectcomposition and impurities can have a detrimental impact on LFPperformance and thus the lithium-ion battery as a whole. Secondly, thevarious synthesis methods utilized by material suppliers can result innon-ideal primary and secondary particle sizes, an average surface areathat is too low, and a particle morphology that can limit the rateperformance of the cathode. An LFP with carefully controlledelectrochemical and physical characteristics is therefore needed toprovide consistent results when incorporated into lithium-ion batteries.

One example of an LFP material to be used as a high power electrodematerial was disclosed by Beck et al in U.S. patent application Ser. No.14/641,172. For example, in U.S. patent application Ser. No. 14/641,172,the active electrode material includes LFP synthesized from aspheniscidite FePO₄ (NH₄Fe₂(PO₄)₂OH*₂H₂O) precursor, herein alsoreferred to as spheniscidite FePO₄-LFP. Utilizing spheniscidite FePO₄ asthe iron phosphate (FePO₄) precursor material resulted in specificparticle morphology with high surface area and enhanced surfacefeatures. These properties resulted in an active electrode material withexceptionally high power, especially at temperatures at 0° C. and below,when compared to other LFP-active electrode materials at lowtemperatures. This spheniscidite FePO₄-LFP demonstrated improved coldcrank performance for low voltage starter, start-stop, and mild-hybridbattery applications. The LFP synthesized from a spheniscidite FePO₄precursor, including the above, discussed properties as disclosed inU.S. patent application Ser. No. 14/641,172, entitled “High PowerElectrode Materials,” filed Mar. 6, 2015, the entire contents of whichare hereby incorporated by reference for all purposes.

The inventors herein have recognized that there are three key reasonsfor further development of the technology described in U.S. patentapplication Ser. No. 14/641,172 titled “High Power Electrode Materials”:(1) to increase the first charge capacity (FCC); (2) to eliminate use ofvanadium in the plus five (+5) oxidation state; and (3) mitigate ammonia(NH₃) emission during the precursor preparation and calcinationprocesses.

Low FCC, when compared to the theoretical capacity of the activematerial, reduces the energy density of the lithium-ion battery.Therefore, an increase in the FCC when compared to current LFPmaterials, for example the LFP synthesized from spheniscidite FePO₄,would improve the overall energy density of the cell without negativelyimpacting the power performance.

Trivalent vanadium is considerably more benign than pentavalentvanadium. Replacing the pentavalent vanadium as described in U.S. patentapplication Ser. No. 14/641,172 with a non-obvious trivalent vanadiumprecursor promotes an FCC increase and NH3 emission decrease, whilemaintaining the rate and low temperature power performance of the LFP.It is additionally necessary to use pollution control systems whenmanufacturing a product that results in a measurable release of NH3.These pollution control systems result in an added manufacturing cost.The increased cost coupled with the commitment to enhance theenvironmentally friendliness of our manufacturing processes, providesignificant drivers to eliminate, or significantly reduce, the NH3emission associated LFP production as described in U.S. patentapplication Ser. No. 14/641,172. As described herein, the inventors haverecognized replacement of spheniscidite FePO4, which is most likely themain precursor contributing to the NH3 emissions, with a non-obviousiron phosphate precursor that results in an FCC increase whilemaintaining the rate and low temperature power performance of the LFP isdesirable both from an economic and safety perspective.

An additional area of development that is a focus of the teachingscontained herein is to mitigate the moisture uptake of the final LFPboth in powder form and when incorporated into an electrode of anelectrochemical energy storage device. Engineering a particle porestructure that can maintain high surface areas, maintain parity in termsof the cumulative pore volume when compared to the prior art, whilesimultaneously shifting the majority of the pores to a diameter on thenanometer scale, can mitigate performance and manufacturing challengesthat have been attributed to elevated levels of moisture in thelithium-ion cell. Elevated levels of moisture uptake in an activematerial can impact lithium-ion battery cell manufacturing as theelectrodes that contain the active material with the high levels ofmoisture may be thermally treated to remove the moisture, kept in a dryenvironment, and then thermally treated again once incorporated into theelectrochemical energy storage device before the liquid electrolyte isadded. The described process requiring multiple thermal treatments addstime and cost to the manufacturing process.

If the moisture from the active material is not effectively removed fromthe energy storage device, the moisture can diffuse through the liquidelectrolyte until in contact with the negative electrode. Once incontact with the negative electrode, the moisture can beelectrochemically reduced thereby forming a gas. Gas formation withinthe cell is not ideal as it causes a pressure increase that can be adetriment to the longevity of the energy storage device. Moisture, onceintroduced into the device, has been demonstrated to react with certainlithium-ion salts utilized in lithium-ion battery electrolytes. Thisreaction results in the formation of corrosive species that degrade theperformance of the device components leading to decreased devicefunction and reduced lifetime. In addition, the corrosive species thatcould be formed by the above demonstration, may contribute to theformation of an electrochemically inactive Li-species. The formation ofthis inactive species accelerates the reduction of the energy storagecapacity of the device and thus detrimentally impacts the device life.

To that end, the inventors herein disclose methods and materials ingeneral including identifying zero NH₃ emission, or low NH₃ emission,formulation(s) of LFP utilizing synthesis methods that provide animproved FCC, maintain high rate capability (defined as a 10 C dischargecapacity of greater than 140 mAh/g at 23° C.), and ensure the lowtemperature performance (defined as a direct current resistance (DCR) ofless than 10 ohm when measured for 20 mAh double layer pouch (DLP) cellat −20° C.). As another non-limiting example, the DCR value may be lessthan 9 ohm. In another non-limiting example, the low temperatureperformance may be less than 8.5 ohm.

The inventors herein have also recognized materials and methods thatfurther improve the FCC and rate capability, as well as reduce moistureuptake (thus resulting in a reduction in gas formation during the lifeof the lithium-ion battery) while maintaining low or no NH3 emission. Inone example, an LFP electrochemically active material for use in anelectrode comprising a phosphate to iron molar ratio of 1.000-1.050:1, adopant comprising vanadium in a trivalent state and optionally aco-dopant comprising cobalt, and a total non-lithium metal to phosphatemolar ratio of 1.000-1.040:1 is provided. As another example, an LFPelectrochemically active material may be provided which may comprise aphosphate to iron molar ratio of 1.020-1.040:1 and a dopant comprisingvanadium in a trivalent state, wherein optionally comprising a cobaltco-dopant and comprising a total non-lithium metal to phosphate molarratio of 1.001-1.020:1. Still, a further example LFP electrochemicallyactive material may comprise a phosphate to iron molar ratio of1.0300-1.0375:1, a dopant comprising vanadium in a trivalent state,optionally comprising a cobalt co-dopant, and a total non-lithium metalto phosphate molar ratio of 1.0025-1.0050:1.

As a specific example, an LFP electrochemically active materialsynthesized from an iron phosphate precursor with an iron weight percentin the range of 28-37 wt. %, and 0-5 dopants wherein one dopant may bevanadium which may be present in the LFP formula within the range of0.0-5.0 Mol. % and one dopant may be cobalt which may be present in theLFP formula within the range of 0.0-1.0 Mol. %. As another non-limitingexample, an LFP material synthesized from an iron phosphate precursorwith an iron weight percent in a range of 35-37 wt. % and 1-2 dopantswherein one dopant may be vanadium which may be present in the LFPformula in a range of 2.0-4.0 Mol. % and one dopant may be cobalt whichmay be present in the LFP formula in a range from 0.0-0.5 Mol. %. Stilla further example of an LFP electrochemically active material may besynthesized from an iron phosphate precursor with an iron weight percentin a range between 36.0 and 37.0 wt. %.

A method to form an LFP electrochemically active material for use in anelectrode, comprises mixing a vanadium dopant in a trivalent state, alithium source, a carbon source, an iron phosphate source with an ironcontent of at least 28 wt. % and a phosphate to iron molar ratio of1.000-1.040:1, and optionally a co-dopant, adding a solvent to form aslurry, milling the slurry, drying the milled slurry to form an LFPprecursor powder, firing the dried powder to obtain the LFPelectrochemically active material, wherein the LFP comprises thevanadium dopant and/or co-dopant partially substituting Fe in a crystallattice structure, a phosphate to iron molar ratio of 1.000-1.050:1, anda total non-lithium metal to phosphate molar ratio of 1.000-1.040:1.

Further, the final LFP powder may have a surface area greater than about25 m²/g within the range of 25-35 m²/g for example. Further, as anon-limiting example, the final LFP powder may have, a tap densitywithin a range of 1.0-1.5 g/mL, and an FCC of greater than 145 mAh/g anda 10 C discharge capacity of greater than 135 mAh/g.

As another example, the final LFP powder may comprise a surface area inthe range of 28-32 m²/g, a tap density within the range of 1.10-1.40g/mL, an FCC of greater than 150 mAh/g, and a 10 C discharge capacity ofgreater than 138 mAh/g. A further example of the final LFP powder maycomprise a surface area in the range of 29-31 m²/g, a tap density in therange of 1.20-1.30 g/mL, an FCC greater than 152 mAh/g, and a 10 Cdischarge capacity of greater than 140 mAh/g.

The general purpose of the present teachings described herein relates tothe physical structure of materials, methods to synthesize thosematerials, methods to identify successful process of said materials, andthe use of those materials in an electrochemical energy storage device.The teachings presented herein are most directly applicable tolithium-ion based electrochemical energy storage devices, but notlimited to such a device if practiced by one skilled in the art. Alithium-ion based electrochemical energy storage device may utilize twoelectrodes, an electrolyte solution, and a porous, electricallyinsulating separator containing said electrolyte that is placed betweenthe electrodes. When constructed in the above described manner, theseenergy storage devices can reversibly store energy through reduction andoxidation reactions that occur in the active materials incorporated intothe electrodes. The previous discussion regarding the active componentschemical composition, electrolyte, overall reactions when energy isbeing stored or released, and the mechanism for which charged speciesare transported within the device is applicable to these teachings aswell.

As provided herein, systems and methods for sustaining large charge anddischarge currents while minimizing the cell capacity are disclosed,especially with the use of lithium-ion battery technology for lowvoltage transportation applications. The requirement has prompted theneed for batteries that can consistently maintain large charge anddischarge current densities while maintaining a high level of safety.

The disclosed embodiments may include manipulating the primary andsecondary particles pore structure such that the total pore volumereaches parity with the teachings described in U.S. patent applicationSer. No. 14/641,172 and further shifting the pore size distribution suchthat a large percentage of the pores are in the sub 10-nm range. Theresult of holding the total pore volume constant while shifting the poresize to smaller diameters reduces the overall moisture uptake. Anotherembodiment discloses optimal unexpected dopant levels as well as thespeciation of the dopants which are effectively incorporated into theLFP crystal structure as measured by an increase in the rate performancealong a wide temperature range. An additional embodiment of this presentdisclosure includes the production and thermal response of the LFPprecursor powder ensuring a final LFP product with the needed physicaland electrochemical attributes to function as a high rate cathode forlithium-ion batteries.

It will be understood that the summary above is provided to introduce,in simplified form, a selection of concepts that are further describedin the detailed description. It is not meant to identify key oressential features of the claimed subject matter, the scope of which isdefined uniquely by the claims that follow the detailed description.Furthermore, the claimed subject matter is not limited toimplementations that solve any disadvantages noted above or in any partof this disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of iron phosphate precursors and dopantprecursors and their contribution to NH₃ emission during LFP synthesis.

FIG. 2 is an illustration of vanadium phosphate synthesis steps.

FIG. 3 is a high magnification SEM image of spheniscidite FePO₄.

FIG. 4 is a high magnification SEM image of a FePO₄*qH₂O sample.

FIG. 5 is a high magnification SEM image of a second FePO₄*qH₂O sample.

FIGS. 6A and 6B are low and high magnification images of pure phase (PP)FePO₄-LFP synthesized using the methods and precursors described herein.

FIG. 7 is a magnified image of PP FePO₄-LFP synthesized using themethods and precursors described herein.

FIG. 8 is a pore size distribution curve of the modified PP FePO₄-LFPsynthesized using the methods and precursors described herein ascompared to spheniscidite FePO₄-LFP.

FIG. 9 is a chart comparing moisture uptake of spheniscidite FePO₄-LFPand PP FePO₄-LFP in powder and electrode forms.

FIG. 10 is a chart comparing moisture uptake of spheniscidite FePO₄-LFPand PP FePO₄-LFP at different exposure times.

FIG. 11. is an example method for the synthesis of LFP with low or noNH₃ release using the disclosed dopant precursor(s).

FIGS. 12A and 12B are interval plots of the various LFP samplesdescribed herein that compare the DCR at room temperature and −20° C.

FIG. 13 is a voltage versus time plot that provides an indication of thecold crank capability of the various LFP samples described herein at−30° C.

FIGS. 14A and 14B are interval plots of the various LFP samplesdescribed herein that compare the DCR, at room temperature and −20° C.

FIGS. 15A, B, and C are plots measuring FCC, 10 C discharge capacity andsurface area of a FePO₄-LFP formulation with a secondary impurity phase.

FIGS. 16A, B, and C are plots measuring FCC, 10 C discharge capacity andsurface area of a PP FePO₄-LFP formulation.

FIG. 17 shows an illustration of thermal zones from a thermal analysis.

FIG. 18 shows an example electrode assembly.

FIGS. 19A and 19B show a wound cell example.

DETAILED DESCRIPTION

The present disclosure will now be described more fully hereinafter withreference to the accompanying drawings, in which exemplary embodimentsof the invention are shown. The particular embodiment(s) is merelyexemplary in nature and is in no way intended to limit the scope of theinvention, its application, or uses, which may, of course, vary. Theinvention is described with relation to the non-limiting definitions andterminology included herein. These definitions and terminology are notdesigned to function as a limitation on the scope or practice of theinvention but are presented for illustrative and descriptive purposesonly. While the processes or compositions are described as an order ofindividual steps or using specific materials, it is appreciated thatsteps or materials may be interchangeable such that the description ofthe invention may include multiple parts or steps arranged in many ways.

Components, process steps, and other elements that may be substantiallythe same in one or more embodiments are identified coordinately and aredescribed with minimal repetition. It will be noted, however, thatelements identified coordinately may also differ to some degree.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting. As used herein, thesingular forms “a,” “an,” and “the” are intended to include the pluralforms, including “at least one,” unless the content clearly indicatesotherwise. “Or” means “and/or.” As used herein, the term “and/or”includes any and all combinations of one or more of the associatedlisted items. It will be further understood that the terms “comprises”and/or “comprising,” or “includes” and/or “including” when used in thisspecification, specify the presence of stated features, regions,integers, steps, operations, elements, and/or components, but do notpreclude the presence or addition of one or more other features,regions, integers, steps, operations, elements, components, and/orgroups thereof. The term “or a combination thereof” or “a mixture of”means a combination including at least one of the foregoing elements.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this disclosure belongs. It willbe further understood that terms such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art and thepresent disclosure, and will not be interpreted in an idealized oroverly formal sense unless expressly so defined herein.

The present disclosure provides an environmentally friendly LFPformulation utilizing low ammonium, or no ammonium, containing precursorspecies as well as replacing dopants precursors with more effective andbenign materials. The disclosed LFP material reduces NH₃ emissions, andcan completely eliminate NH₃ emissions, as illustrated in FIG. 1. Thisis in contrast to previous LFP synthesis methods and precursors asdescribed in U.S. patent application Ser. No. 14/641,172. The LFPdescribed herein is synthesized from dopants comprising safer metalions, such as a trivalent vanadium ion. The trivalent vanadium ion maybe provided as vanadium phosphate, for which an example synthesis methodis illustrated in FIG. 2. The LFP also includes different FePO₄precursors that were indicated in FIG. 1. These FePO₄ precursors havedifferent crystalline structures and showed different morphology asshown in FIG. 3, FIG. 4, and FIG. 5. Using the different FePO₄precursors, when compared to the prior art, did not negatively impactthe primary particle size or the overall LFP particle morphology asillustrated in FIG. 6A, FIG. 6B, and FIG. 7. The different precursorsand synthesis methods did, however, contribute to a beneficial change inthe pore size distribution within the particles as illustrated in FIG.8. The control of the pore size, pore size distribution, and pore volumemay help in decreasing moisture uptake as shown in FIGS. 9 and 10.Synthesizing an LFP (as shown in FIG. 11) using the FePO₄ precursordescribed herein may result in the improved properties as displayed inFIGS. 12-16, with regards to FCC, DCR, surface area, 10 C capacity, etc.The high purity of the precursor material is important, and FIG. 17further shows a thermal profile of said LFP precursor powder that mayresult in the aforementioned improved properties. As one example, theLFP precursor may experience a thermal weight loss of less than 40%,more specifically less than 30% and more specifically, less than 25%.Thus, an iron phosphate with no NH₃ release during synthesis, may beused to synthesize the LFP including a trivalent vanadium dopant toproduce a high performance LFP, while also decreasing moisture uptake,and further may be incorporated into electrochemical cells, as shown inFIGS. 18 and 19.

In order to minimize, or completely eliminate, the NH₃ emission from thesynthesis process, thereby improving the process described in the priorart, the source(s) of the emission and viable replacement candidates maybe identified. This analysis is highlighted in the illustration shown inFIG. 1.

Turning to FIG. 1, the LFP as described herein may have two sources ofNH₃ emissions. In other examples, additional sources of NH₃ emissionsmay be present and additional replacement candidates may be identified.As shown in schematic 100, the first and most significant source is theFePO₄ precursor and the second is the dopant precursor. As demonstratedby the illustration, when spheniscidite FePO₄, (NH₄Fe₂(PO₄)₂OH*2H₂O), isutilized as the FePO₄ source, significant NH₃ may be released. When thespheniscidite FePO₄ is replaced with FePO₄*qH₂O, where q can vary fromapproximately 0 to 2, the NH₃ emission from the FePO₄ source iseliminated. In one example, the FePO₄*qH₂O may be a pure-phase FePO₄ (PPFePO₄) or a FePO₄ comprising a secondary impurity phase (SP FePO₄). Thesecond source of NH₃ emission is associated with the ammonium vanadate(NH₄VO₃) dopant. The NH₃ may be eliminated by utilizing VPO₄, achemically compatible species with LFP that is not commerciallyavailable and for which the novel synthesis is described as anembodiment of this present disclosure. In order to achieve the targetedLFP performance, as articulated herein, a cobalt based co-dopant hasbeen investigated at low concentrations. While this precursor has thepotential to release NH₃ during the LFP synthesis process, the releasemay be negligible and dramatically less when compared to the LFPsynthesis described previously for current LFP materials. Alternatively,a cobalt precursor that does not release NH₃ has also been identifiedand is illustrated in FIG. 1. All different LFP samples that wereinvestigated for this work used the FePO₄*qH₂O (PP FePO₄ or including SPFePO₄), combined with NH₄VO₃ or VPO₄ dopant, and in the presence orabsence of a cobalt based co-dopant.

The low NH₃ LFP method described herein includes iron phosphateprecursors which comprise no ammonium in the formula that can besubsequently reduced to NH₃ during the synthesis process. The LFPsynthesized from the iron phosphate precursor material further includesa P/Fe ratio of 1.000-1.050:1 in the final LFP powder. Additionally, thelow NH₃ LFP method can include multiple dopant formulations as listed atFIG. 1. The dopant may substitute for the Fe in the LFP crystal latticestructure. The electrochemical performance and physical characteristicsfor LFP synthesized with varying vanadium dopant precursorconcentrations, vanadium dopant precursor species, and the addition of acobalt based co-dopant with FePO₄*qH₂O, where q can vary fromapproximately 0 and 2, are highlighted in Table I and Table II below.

TABLE I Example dopant formulations for low NH₃ emission LFP methodDischarge Carbon Tap Surface Capacity Dopant Content Density Area FCC(mAh/g) (Molar %) (%) (g/ml) (m²/g) (mAh/g) 0.2C 10C Ex. 1 4% VPO₄ 2.31.32 29.83 144.5 153.8 145.2 0.25% NH₄CoPO₄ Ex. 2 4% VPO₄ 2.7 1.49 20.26150.8 153.4 124.5 0% NH₄CoPO₄ Ex. 3 4% NH₄VO₃ 2.6 1.56 19.57 150.9 151.9117.0 0% NH₄CoPO₄ Ex. 4 3% NH₄VO₃ 2.8 1.59 19.91 148.9 150.3 117.7 0%NH₄CoPO₄

There are a number of key, unexpected findings from the variation insynthesis precursors. Thus, simply replacing the spheniscidite FePO4 andammonium vanadate with FePO4*qH₂O, where q is not optimized, andvanadium phosphate may not consistently yield the improved rateperformance as outlined herein. While the target FCC and elimination ofNH3 emission from the synthesis may be achieved, the rate performancemay not be as high as demonstrated in Ex. 2 highlighted in Table I.Additionally, replacing the spheniscidite FePO4 with FePO4*qH2O, where qis not optimized, while utilizing the ammonium vanadate as the dopantprecursor may also result in an LFP powder with unsatisfactory physicalproperties and electrochemical performance as highlighted in Table I,Ex. 3 and Ex. 4. It was unexpectedly found that a combination of theFePO4*qH2O, where q is not optimized, vanadium phosphate, and a cobaltbased co-dopant included in the formulation resulted in the improved LFPmaterial as demonstrated by Ex. 1 in Table I. For example, at the lowmolar percentage outlined in Table I, a substantial increase in the LFPsurface area and rate capability was observed using 0.25% NH4CoPO4 incombination with the specific iron phosphate and vanadium source. Thisresult demonstrates that a co-dopant approach in which the minoritydopant is cobalt based with ammonium as a constituent of the precursormolecule increases the LFP particle surface area by approximately 50% aswell as resulting in a 10 C discharge capacity greater than 140 mAh/g.However, the target FCC of greater than 150 mAh/g may not be achieved.In other examples, other cobalt based ammonium precursor molecules mayincrease the LFP material properties such that all of the target metricsare achieved.

Another potential iron phosphate precursor as highlighted in FIG. 1 thateliminates NH₃ emissions from the iron phosphate precursor during thesynthesis process is FePO₄*qH₂O, where q is optimized, which also has aP/Fe ratio of 1.000-1.050:1 in the final LFP powder. As described inTable II below, this precursor was also utilized with multiple dopantspecies and formulations in order to attain the desired LFP physical andelectrochemical characteristics described herein. As in the previouslydescribed synthesis with FePO₄*qH₂O, where q is not optimized, thedopant is substituting the Fe in the LFP crystal. The electrochemicalperformance and physical characteristics for LFP synthesized withvarying vanadium dopant precursor concentrations and vanadium dopantprecursor species with FePO₄*qH₂O, where q may vary from approximately 0and 2 are highlighted in Table II.

TABLE II Electrochemical performance and physical characteristics forLFP with varying vanadium dopant precursor concentrations DischargeCarbon Tap Surface Capacity Dopant Content Density Area FCC (mAh/g)(Molar %) (%) (g/ml) (m²/g) (mAh/g) 0.2C 10C Ex. 1 4% NH₄VO₃ 2.7 1.2624.19 150.9 155.3 141.6 Ex. 2 4% VPO₄ 2.8 1.34 26.16 152.6 157.7 142.6Ex. 3 2% NH₄VO₃ 2.4 1.39 21.49 152.2 157.5 138.8 Ex. 4 2% VPO₄ 2.3 1.3723.64 152.6 158.4 141.0

When using FePO₄*qH₂O, where q is optimized, and VPO₄ at a dopantconcentration equivalent to that utilized in the spheniscidite FePO₄-LFPsynthesis, the room temperature performance in terms of the FCC issuperior when compared to spheniscidite FePO₄-LFP and the 10 C dischargeare above the target value as demonstrated by Ex. 2 in Table II. Ascarbon content and tap density values are in a comparable range, thesurface area, however, is below target which results in a DCR at −20° C.greater than the target resistance. Thus, the combination of FePO₄*qH₂Oand the vanadium dopant described above as precursors for LFP productionsignificantly reduce and/or eliminate NH₃ emissions while coming closeto meeting all of the target requirements as articulated herein. As anexample, the FePO₄*qH₂O where q may range between 0 and 2 and whereinthe water may be present in the range of 0.0-20 wt. % may be provided.In another example, where q may be 0, the water may be present at lessthan 5 wt. %. In another example, the ammonium vanadate can be replacedby vanadium phosphate, thereby completely eliminating the NH₃ emissionfrom the LFP synthesis and replacing the pentavalent vanadium with themore benign trivalent vanadium. As demonstrated by Table II, allelectrochemical properties satisfy the above target values and allphysical properties satisfy the target specifications with the exceptionof the surface area. The electrochemical performance coupled with thereduced NH₃ emissions and increased environmental friendliness of thesynthesis process demonstrates the novelty of this synthesis method andprecursor choice. It was found that while replacing the vanadium dopantsource with VPO₄, the surface area was improved, albeit lower than thetarget, while maintaining the superior FCC and 10 C discharge capacityof greater than 150 mAh/g and 140 mAh/g, respectively. This may be dueto increased dopant efficiency through a more homogeneous distributionof the vanadium throughout the LFP crystal structure.

In one example, the V dopant precursor is VPO₄ because it does notcontain ammonium and is more benign than other commonly used vanadiummetal ion dopants, such as pentavalent vanadium dopant. However, VPO₄ isnot currently commercially available because previous syntheses arecomplicated, costly, and difficult to scale up.

As shown in FIG. 2, an example method 200 of synthesizing VPO₄ isillustrated. The disclosed method of VPO₄ synthesis involves thefollowing chemical reaction:

V₂O₅+2NH₄H₂PO₄+C→2VPO₄+2NH₃+3H₂O+CO₂

During the reaction, V⁵⁺ is reduced to V³⁺ to form VPO₄. The materialprocessing for this reaction includes, at 202, pre-mixing precursors insolvent, at 204, stirring the slurry at an elevated temperature whileadding a carbon source or reducing agent such as sugar, citric acid(CA), glucose or others. For example, a carbon source may comprise anyorganic carbon source that is at least moderately soluble in thereaction solvent such as glycol or PVB. Further, the solvent maycomprise water or an organic solvent such as an alcohol. At 206, theslurry is milled for a moderate time, and at 208, the milled slurry isdried into powder form. At 210, the abovementioned chemical reductionutilizing a temperature programmed reaction (TPR) takes place as thepowder is fired under an inert atmosphere. Once fired under an inertatmosphere, a high purity VPO₄ compound may be obtained at step 212. Asan example of preparing VPO₄, precursors of V-oxide compound andphosphate source compound were mixed in solvent with slight heatingwhere the slurry was stirred for 10-16 hours. As an example, vanadiumprecursors may comprise vanadium oxides and/or vanadate precursorspecies. The milled slurry was then spray dried and the powder wasconverted to VPO₄ by a TPR under an inert gas in a tube furnace. In someexamples, the firing gas may comprise any noble gas or a mixture thereofsuch as N₂, Ar, and N₂/Ar. The phosphate source may comprise any specieswith a phosphate anion that is at least moderately soluble in thereaction solvent. For example, the phosphate source may comprisephosphoric acid, NH₄H₂PO₄, and (NH₄)₂HPO₄, or a combination thereof. Insome examples, carbon may be present in the VPO₄ at less than 2.0 wt. %,less than 1.0 wt. %, and even less than 0.5 wt. %. The TPR profile mayinclude ramping from room temperature and then heating to a specifictemperature that may be used to complete the reaction conversion to formVPO₄. The TPR may further include programmed holds at specifictemperatures. Furthermore, in the current disclosure it is taught thatadditional modifications increased the LFP performance while mitigatingNH₃ emission during the synthesis process and moisture uptake when inthe powder and the electrode form.

An example of the different FePO₄-LFP materials discussed above is shownin FIGS. 6A, 6B, and 7. Specifically, FIG. 6A is an SEM image thatdepicts the secondary particles of the PP FePO₄-LFP as disclosed herein.FIG. 6B is a higher resolution image that illustrates the morphology ofthe primary particles of PP FePO₄-LFP. In one example, the secondaryparticles may range from, using d₅₀ as the metric, 1-20 μm in oneexample, and 5-13 μm in another example. The primary particles may rangefrom 25-250 nm in one example, and 25-150 nm in another example. In onespecific example, the primary particles may comprise a size of less than100 nm. Still, a further example may comprise primary particlescomprising a size of less than 80 nm.

The FePO₄ precursor material discussed above may be used to form anelectrochemically active LFP for use in an electrode, via the method asdescribed in FIG. 11, for example. As shown in Table III below, the LFPmay be formed by an above described FePO₄, a dopant, a co-dopant, acarbon source, and a lithium source, wherein the synthesized LFP has aformula that corresponds to Li_(z)Fe_((1-x-y))V_(x)Co_(y)PO₄ where z isgreater than or equal to 1, x is greater than or equal to 0, and y isgreater than or equal to 0. The FePO₄ may be PP FePO₄ or may include SPFePO₄, and may have an Fe content of at least 25 wt. %, or at least 30wt. %, in another example. In a further example, the Fe content may bebetween 28-37 wt. %. Further, the FePO₄ precursor may have a phosphateto iron molar ratio as close to unity as possible. For example, FePO₄precursors may be provided wherein the phosphate to iron molar ratiosare within the ranges of 1.00-1.04:1, 1.00-1.02:1, or 1.00-1.01:1.Further, the phase impurity of the FePO₄ may be less than 10%, less than5% in some examples, or less than 3% in other examples. Providing aphosphate to iron ratio as close to unity as possible is an indicationof phase purity which may result in improvements in formula compositionsand lessens the amount of precursor material needed to achieve anoptimal composition, for example.

Further as depicted in Table III, the above described FePO₄ precursorused in making the lithium iron phosphate may have a surface area of 10to 40 m²/g and the subsequent lithium iron phosphate precursor powdermay have a thermal profile as illustrated in FIG. 17. As an example,other FePO₄ precursors are considered wherein the surface area may bewithin the ranges of 10-30 m²/g or between 10-20 m²/g.

A dopant may further be included in the electrochemically activematerial. In one example, VPO₄ is included. Additionally, a cobaltco-dopant may be added. In one example, the cobalt co-dopant may beNH₄CoPO₄. In another example, the cobalt co-dopant may be CoC₂O₄. Anon-NH₃ emission synthesis approach may include, for example, the abovedescribed FePO₄ as depicted in Table III (PP FePO₄), VPO₄, and CoC₂O₄. Alow NH₃ emission synthesis approach may include, for example, FePO₄ asdepicted in Table III (PP FePO₄), VPO₄, and NH₄CoPO₄. Furthermore, alithium source and carbon source may be added to synthesize theelectrochemically active material, and the final LFP powder may have asurface area between 25 to 35 m²/g. In some examples, the lithium sourcemay comprise Li₂CO₃, LiH₂PO₄, or any other suitable lithium source. Itwill be appreciated that the lithium sources are provided as exemplaryspecies and that any suitable lithium source may be used. The totalnon-lithium metal to phosphate ratio may range from 1.000-1.040:1 in oneexample, or 1.001-1.020:1 in another example. As a further non-limitingexample, the total non-lithium metal to phosphate ratio may range from1.0025-1.0050:1. The above ranges demonstrated high cell performance, asdiscussed further below. Additional compatible substances may be addedto achieve the disclosed ratios, the technique for which is known to aperson of ordinary skill in the art.

In order to understand V—Co co-dopant effects on electrochemical energystorage performance, a group of LFP laboratory samples were synthesizedwith PP FePO₄ precursor and 0.0-5.0 Mol. % VPO₄ and 0.0-1.0 Mol. %NH₄CoPO₄ or CoC₂O₄ as a V—Co co-dopant precursors. In one example, theLFP electrochemically active material in the current disclosure maycomprise a vanadium dopant, such as the vanadium dopant synthesized inFIG. 2. It will be appreciated that in at least one example, thevanadium dopant may be contributed by an oxyanion species such as anoxide, carbonate, oxalate, phosphate, or other suitable sources forwhich vanadium is considered the cation. In one example, the vanadiumdopant source may comprise one or more of VPO₄ and NH₄VO₃. Additionally,the electrochemically active material may comprise a cobalt co-dopant,such as CoC₂O₄ or NH₄CoPO₄, at 0.0 to 0.5 Mol. %.

TABLE III Characteristics of FePO₄ *qH₂O; dopant; co-dopant (optional);lithium source example, LFP precursor power; and LFP final powder RANGEFePO₄ *qH₂O 0 ≤ q ≤ 2, approximately Fe content 28 to 37 wt. % P/Feratio 1.000-1.040 Surface Area 10 to 40 m²/g Dopant VPO₄ or NH₄VO₃0.0-5.0 Mol. % Co-Dopant NH₄CoPO₄ or CoC₂O₄ 0.0-1.0 Mol. % LithiumSource Li₂CO₃; LiH₂PO₄; or other lithium LFP Precursor Powder sourceThermal Profile LFP Final Powder 3 key thermal peaks at 75-125° C., 175-250° C., and at 275-425° C. Total non-lithium metal 1.000-1.040 tophosphate ratio Total P/Fe ratio 1.000-1.050 Surface Area 25-35 m²/gParticle size Secondary: d50: 1-20 μm; optimal 5-13 μm; Primary: 25-150nm; preferred: <100 nm optimal: <80 nm

As noted above, one of the findings of the LFP cathode material coveredin U.S. patent application Ser. No. 14/641,172 is that of high wateruptake. Spheniscidite FEPO₄-LFP, with its high surface area and highconcentration of pores with an average diameter on the order of 100-500nm, may be sensitive to water. This uptake may occur even at low levelsof exposure, which utilizes more stringent control of the environmentduring the handling and processing of this material. Processingincludes, but is not limited to, electrode coating, electrode stamping(or other such handling process), and cell assembly. A concernassociated with water presence in a final cell at appreciableconcentrations is the production of hydrogen gas due to electrochemicalreduction of water at the anode, or negative electrode. In addition, thepresence of moisture in the cell may react with various constituents ofthe electrolyte to form other by-products, including HF, which may causedissolution of the metal in the cathode and/or the metal in the currentcollectors and therefore degrade cell performance.

Having a high surface area cathode material for lithium ion batteryapplications is preferred, and in fact may be a key characterizationmetric. High water uptake, in which the absolute quantity can increasewith an increase in surface area due to more active sites for wateruptake, is a concern. In order to address this concern, an optimizedparticle interior structure for the LFP can mitigate the water uptake.The particle structure of interest in this case is pore size, total porevolume, and the pore size distribution. In some examples, the majorityof the pores may comprise a size of less than 150 nm, less than 50 nm,or less than 15 nm. In one example, if the total pore volume normalizedby the mass of the powder for a given set of LFPs is equivalent, and thepore size can be decreased with the majority of the pores confined to adiameter of approximately 10 nm or less, an appreciable decrease in themoisture uptake may occur. For example, the pore size distribution ofspheniscidite FePO₄-LFP showed the presence of two pore diameter ranges,the first was centered at a diameter of approximately 2.5 nm while theother occurred over a broad range diameter range of 10-100 nm. The totalpore volume normalized to the mass is calculated by integrated the areaunder the pore size distribution curve as showed in FIG. 8 and has beenmeasured at 0.19 cm³/g. In some examples, the cumulative pore volume maycomprise a value of greater than 0.08 cm³/g. In other examples,cumulative pore volume may be greater than 0.15 cm³/g. As anothernon-limiting example, the cumulative pore volume may be greater than0.18 cm³/g.

Specifically, as shown in FIG. 8, chart 800 depicts a gravimetricallynormalize pore volume in units of (cm³/g×nm) as a function of porewidth, which can be indicated as the pore diameter. Pore sizedistribution is shown at 802 for spheniscidite FePO₄-LFP and at 804 forPP FePO₄-LFP. At around 2.5 nm pore width, PP FePO₄-LFP exhibits alarger pore volume compared to spheniscidite FePO₄-LFP. Conversely, atabout 20 nm pore width, spheniscidite FePO₄-LFP exhibits a larger porevolume compared to PP FePO₄-LFP. In one example, the PP FePO₄-LFP maythus exhibit a different overall structure than that of sphenisciditeFePO₄-LFP, wherein PP FePO₄-LFP comprises generally smaller pores. Assuch, PP FePO₄-LFP may exhibit different moisture uptake properties, asdiscussed further below. It is noted that although there are significantdifferences in pore size distribution, an equivalent cumulative porevolume is held for both species.

As shown in Table IV below, the equivalent cumulative pore volume allowsfor PP FePO₄-LFP to have a surface area consistent with that ofspheniscidite FePO₄-LFP. Moreover, the carbon amount (%) and tap densitymay be consistent as well. Thus, PP FePO₄-LFP may maintain theadvantages associated with high surface area and total pore volume inrelation to power performance, while also exhibiting a pore sizedistribution that may mitigate moisture uptake. Furthermore, thesimilarity of tap density and surface area as well as the carbon contentbetween the samples eliminates these factors as causes for the observedmoisture uptake differences.

TABLE IV Example comparison of spheniscidite FePO₄-LFP and PP FePO₄-LFPTap Surface Sample Density (TD) Area (m²/g) Carbon % spheniscidite1.1-1.5 27-31 2.5 ± −0.4 FePO₄-LFP PP FePO₄-LFP 1.2-1.4 25-35 2.5 ± −0.4

The moisture analysis showed a reduced total moisture uptake in bothpowder and electrode forms at different controlled exposure time for aPP FePO₄-LFP when compared to spheniscidite FePO₄-LFP, as shown in FIGS.9 and 10. The samples were dried in a vacuum oven at 85° C. forapproximately 18-24 hours. The samples were kept in the dry room, andsamples were analyzed after 2 hr, 1 week, and 10 days using Karl Fisherequipment and processed at 220° C. A reduction in the absolute moistureuptake is an advantageous material structure improvement as it increasesmaterial stability, performance, and safety. As an example, lithium-ioncells that were built and tested with higher water content (Examplecells 1 and 2 in Table V) experienced an increase in the total gasproduction when in the charged state. Analysis of this gas showed a highhydrogen gas concentration while a lithium-ion cell built with a reducedwater concentration in the electrode (Example Cell 3 in Table V) did notproduce a measurable amount of gas and the measured hydrogen gasconcentration was approximately an order of magnitude lower.

TABLE V Gas analysis of lithium-ion cells that were built with high andlow moisture High Moisture Low Moisture Example Cell 1 Example Cell 2Example Cell 3 1^(st) 2^(nd) 1^(st) 2^(nd) 1^(st) 2^(nd) Gas Analysisdraw draw draw draw draw draw Hydrogen 41.6% 45.9% 31.4% 29.3% 2.9% 3.9%

As shown in FIG. 9, data with relation to moisture uptake is provided inchart 900, which compares the moisture uptake between sphenisciditeFePO₄-LFP and PP FePO₄-LFP in electrode and powder forms. Specifically,the electrode comprising PP FePO₄-LFP is at 902 and the PP FePO₄-LFPpowder is at 904. The electrode comprising spheniscidite FePO₄-LFP is at906, and the spheniscidite FePO₄-LFP powder is at 908. As shown, themoisture uptake of spheniscidite FePO₄-LFP is about 30-40% higher thanthat of PP FePO₄-LFP. Moisture uptake is believed to be related to thestability of the cell, e.g., less moisture uptake is important forperformance stability and low gassing. PP FePO₄-LFP shows less wateruptake, and thus may experience less gassing and significantly lesshydrogen production. In part due to lower moisture uptake, PP FePO₄-LFPis easier to process from a production stand-point and may show improvedstability. In one example, the smaller pore size distribution of the PPFePO₄-LFP changes the structure of the LFP and mitigates moistureuptake, which makes the material easier to handle and enhances cellperformance. In one example, an electrochemical cell comprising PPFePO₄-LFP, may mitigate hydrogen production such that it constituentsless than 5% of the gas phase volume or less than 10% in anotherexample. In comparison, an electrochemical cell comprising known LFPformulations may have a hydrogen concentration in the gas phase higherthan 30%. In this way, PP FePO₄-LFP as disclosed may not havesignificant gassing and comprises a low or negligible hydrogenconcentration in the gas phase.

FIG. 10 also depicts moisture uptake comparison in chart 1000. As shown,spheniscidite FePO₄-LFP 1002 showed a significantly higher moistureuptake in a dry room than PP FePO₄-LFP 1004 as a function of exposuretime. Additionally, the data demonstrate that the both species uptake asignificant portion of the total moisture in the initial exposure time,however, that concentration is significantly higher for sphenisciditeFePO₄-LFP.

Further development of LFP composition optimization in the currentdisclosure focuses on the performance-composition relationship of LFPcathode materials synthesized with the PP FePO₄. As previouslydiscussed, low temperature performance is an important parameter forcomposition optimization since it is critical for lithium-ion starterbatteries, start/stop battery applications, and other low voltagelithium-ion battery automotive applications.

Turning to FIG. 11, an example method 1100 is outlined for the synthesisof LFP from an iron phosphate source described herein. The final LFPmaterial may be formed by combining a lithium source, dopant source,carbon source, and the iron phosphate source in a solvent by mixing,milling, drying, and promoting a chemical reduction with a TPR under aninert atmosphere such as N₂. The resulting LFP active material may thenbe useable as a cathode in an electrochemical cell.

At 1102, the method may include mixing an iron phosphate source, alithium source, dopant source and a carbon source in a solvent to form aslurry. In one example, the lithium source may be Li₂CO₃ or LiH₂PO₄. Inone example, the iron phosphate source may be FePO₄*qH₂O. In yet anotherexample, the iron phosphate source may be the iron phosphate source asshown in Table III, e.g., a PP FePO₄ or an SP FePO₄, with an Fe contentof 28 to 37 wt. % and a P/Fe molar ratio of 1.000-1.040:1. Further, theiron phosphate source may have a surface area of 10 to 40 m²/g. In oneexample, the solvent may include an alcohol. In another example, thesolvent may include water. Thus, the method may include an organicsolvent or water based (aqueous) slurry. In one example, the dopantsource may be VPO₄, such as the one synthesized in method 200. Inanother example, the dopant source may be NH₄VO₃. Further, the dopantsource may include a co-dopant source. As such, a co-dopant may also bemixed into the slurry, wherein the co-dopant may be NH₄CoPO₄. In anotherexample, the co-dopant may be CoC₂O₄. Further, the slurry may containabout 0.0-5.0 Mol. % vanadium source (dopant) and about 0.0-1.0 Mol. %co-dopant. A carbon source or more than one carbon source may beincluded at 1103. Once the iron phosphate source, the lithium source,the dopant source, and the carbon source are mixed in a solvent, at1104, the method may include milling the mixture of 1102/1103.

At 1106, the method may include drying the milled mixture of 1104 toobtain an LFP precursor powder. The mixture may be dried using a varietyof methods known to the industry. In one example, the LFP precursorpowder may comprise a thermal profile with three major thermal zones asdiscussed with regard to FIG. 17.

At 1108, the method may include firing the dried material of 1106. Thematerial may be fired to convert the material to the desired LFP by TPR.The TPR may be run in an inert atmosphere, for example N₂. The driedpowder may be converted to the desired LFP by a TPR in N₂ flow in a tubefurnace, a roller hearth kiln, or a rotary calciner for example. The TPRprofile may include ramping from room temperature and then heating. TheTPR may further include programmed holds at specific temperatures. At1110, the method may obtain the desired LFP.

FIGS. 12A and 12B illustrate DCRs at room temperature and −20° C.measured from DLP cells containing PP FePO₄-LFP samples using NH₄CoPO₄and CoC₂O₄ as Co dopant precursors. The DCR at room temperature and −20°C. measured from DLP cells, with spheniscidite FePO₄-LFP and a prior artLFP powder are also listed as references. At room temperature, PPFePO₄-LFP samples using NH₄CoPO₄ as the Co dopant precursor showedcomparable DCR results as that to spheniscidite-FePO₄-LFP. At −20° C.,PP FePO₄-LFP samples using NH₄CoPO₄ as Co dopant precursors showed alower DCR than that of PP FePO₄-LFP samples using CoC₂O₄ as the Codopant precursor indicating a more robust sample for higher powercapability at −20° C., although the DCR of both PP FePO₄-LFP samples metthe low temperature DCR performance target of less than 9 ohm for 20 mAhDLP cells at −20° C. In this specific example, the dopant efficiency ofNH₄CoPO₄ may be higher than that of CoC₂O₄ because NH₄CoPO₄ has asimilar molecular structure to FePO₄ bulk materials. It should beappreciated, in other examples, that depending on the Fe phosphate, theCoC₂O₄ may be optionally preferred depending on the overall system.

Additional results associated with this current disclosure are co-dopantincorporation and how the chemical structure of the dopant impactsdoping efficiency. Specifically described herein is an amorphousNH₄CoPO₄ that is chemically compatible with the final LFP product, has ahigh surface area, and is readily dispersed in the synthesis solvent.These three attributes result in a high doping efficiency that ensureseffective and homogenous incorporation of the dopant even at lowconcentrations. The vanadium phosphate, a crystalline material, is lesssoluble in the synthesis solvent. However, because the molecularstructure of VPO₄ is comparable to that of FePO₄, increased dopantefficiency is achieved when compared to that of ammonium vanadate. Thus,chemical compatibility, especially associated with the anion, may be adriving force for effective dopant incorporation.

Transition-metal-ion doped LFP has been reported in the literature as aneffective route to enhance mass transport of charged species through theLFP crystal structure. This enhanced transport typically results in anelevated level of higher power when compared to non-doped LFP if thedoping efficiency of the dopant is high. Effective doping can result ina homogenous distribution of the dopant throughout the LFP crystalstructure. This homogenous distribution, in conjunction with thetransition metal-ion radius mismatch between the iron and the dopant,may result in impeded LFP crystalline growth during thesintering/calcination process, thereby resulting in smaller LFP crystalswith high surface area, smaller grain boundaries, and the desiredsurface features. As previously described by the above teachings, thechoice of dopants, precursor materials, and processing/synthesistechniques are believed to result in effective doping, which results inan LFP powder similar to that of spheniscidite FePO₄ LFP without the NH₃emissions and a more environmentally benign vanadium precursor.

Vanadium-doped LFP has been previously used to enhance Li-ionconductivity (for example see U.S. Pat. No. 7,842,420 titled “ElectrodeMaterial with Enhanced Ionic Transport Properties”). CoC₂O₄, V₂O₅, andNH₄VO₃ have also been used as dopant precursors for synthesis of dopedLFP. In this current disclosure, NH₄CoPO₄ and VPO₄ were chosen asnon-obvious dopant precursor molecules because they have a similarchemical structure to FePO₄. This similar structure may result in ahigher degree of chemical compatibility because all of the species sharea common anion which may lead to higher dopant efficiency. Anotherreason for using VPO₄ as the dopant precursor of choice is that V (III)is more benign than V (V), as discussed previously.

In addition to an enhanced chemical compatibility resulting in higherdoping efficiency, the morphology and crystalline structure of the FePO₄precursor may also play an important role in attaining the neededphysical and electrochemical properties of above described synthesizedLFP. When investigating the three different FePO₄ precursor materialsutilizing Scanning Electron Microscopy (SEM), it is clear that thespeniscidite FePO₄ and FePO₄*qH₂O precursors with an optimized watercontent have a different nanoscale morphology, as illustrated in FIGS.3-5. This distinction along with the different crystalline structures ofthe above FePO₄, could contribute to the resulting electrochemicalperformance of the LFP. It is therefore non-obvious that the FePO₄precursor morphology, such as that observed with FePO₄*qH₂O with theoptimized water content, is utilized as criteria for precursor selectionwhile simultaneously considering the use of a chemically compatibletrivalent vanadium phosphate as the dopant precursor species. Closeinvestigation of the diffraction patterns clearly demonstrate that thecrystalline system of the optimized FePO₄*qH₂O is hexagonal while thespheniscidite FePO₄ and the non-optimized FePO₄ crystalline systems aremonoclinic. Specifically, different crystalline structures of FePO₄ maylead to different and/or enhanced characteristics. As highlighted above,efficient dopant incorporation unexpectedly may result in smaller LFPcrystalline size and grain boundaries due to metal-ion radius mismatch.This smaller size coupled with homogeneous dopant incorporation mayenhance the electrochemical performance of the resulting LFP powder,which is demonstrated by the data tabulated in Table II.

When considering the non-optimized FePO₄*qH₂O as the main precursormaterial while simultaneously replacing the ammonium metavanadate dopantwith VPO₄, it is demonstrated herein that a high surface area may not bemaintained and the power performance may suffer; albeit the target FCCis achieved. However, a very significant increase in surface area andboost in power performance may be realized by adding NH₄CoPO₄ as aminority co-dopant with as little as 0.5% on a molar basis for example.This result is attributed to increased dopant incorporation into thefinal system as the NH₄CoPO₄ and VPO₄ molecular structures are morechemically comparable to that of FePO₄. An additional consideration isthat the negligible release of NH₃ due to the decomposition of thecobalt dopant precursor may facilitate the synthesis of a smallerprimary particle size through a similar mechanism believed to be in playwhen using speniscidite as the FePO₄ precursor, even though the amountof NH₃ released is significantly lower. It will be appreciated that inat least one example, the cobalt dopant may be contributed by anoxyanion species such as an oxide, carbonate, oxalate, phosphate, oranother suitable source for which cobalt is considered the or one ofmultiple cations in the ionic species. In one example, the cobalt dopantmay comprise one or more of CoC₂O₄, and NH₄CoPO₄.

Turning to FIG. 13, power response is measured from a DLP, as describedabove, using a cold crank test at −30° C. shown in 1300. 1302 indicatesPP FePO₄-LFP with a VPO₄—NH₄CoPO₄ co-dopant, 1304 indicatesspheniscidite FePO₄-LFP, 1306 indicates PP FePO₄-LFP with a VPO₄—CoC₂O₄co-dopant, and 1308 indicates a prior art FePO₄-LFP formulation. Asshown, both PP FePO₄-LFP formulations have similar performance to thatof spheniscidite FePO₄-LFP. In one example, the formulations for 1302and 1306 may include ranges shown in Table III and may be synthesized asdescribed in method 1100.

FIG. 14A shows an interval plot comparing the DCR of 20 mAh DLP cell atroom temperature between a PP FePO₄-LFP with VPO₄—NH₄CoPO₄, an SPFePO₄-LFP (SP FePO₄-LFP) with VPO₄—NH₄CoPO₄, a prior art FePO₄-LFPformulation, and a spheniscidite FePO₄-LFP. At room temperature, the PPFePO₄-LFP displays a DCR that may be comparable to that of sphenisciditeFePO₄-LFP and the SP FePO₄-LFP. The DCR as shown in FIG. 14B also showthat a PP FePO₄-LFP and the SP FePO₄-LFP including a secondary impurityphase have similar results to spheniscidite FePO₄-LFP at −20° C.Overall, the PP FePO₄-LFP displays lenience, stability and robustnesswith a performance comparable to that of spheniscidite FePO₄-LFP yetwith the absence of NH₃ emissions during production, higher FCC, andwith a more environmentally friendly dopant.

FIG. 15A displays the FCC of different samples of an SP FePO₄-LFP. FCCwas measured at FIG. 15A, 10 C discharge capacity was measured at FIG.15B, and surface area at FIG. 15C. Overall, the results showed goodreproducibility. However, in some examples, the SP FePO₄-LFP may haveneed for moisture control during material processing.

FIG. 16A displays the FCC of different samples of a PP FePO₄-LFP. FCCwas measured at FIG. 16A, 10 C discharge capacity was measured at FIG.16B, and surface area at FIG. 16C. Overall, the results were comparableto those of the SP FePO₄-LFP. Furthermore, the results showed highconsistency and reproducibility without the need for extra moisturecontrol due at least in part to the PP FePO₄ having high moistureresistance which may make the synthesis process more controllable.

With respect to the SP FePO₄, in one example, the SP FePO₄ may includeone or more impurities. For example, the impurities may correspond tophases of goethite (Fe⁺³O(OH)) and iron phosphate (Fe(PO₃)₃). Incontrast, PP FePO₄ has a diffraction pattern in which all peaks arewell-defined and can be assigned to FePO₄*qH₂O where q is optimized.This diffraction pattern is indicative of high phase purity. In oneexample, the PP FePO₄ of this disclosure may have a moisture uptake of3% or less even when exposed to high levels of atmospheric moisture forextended periods of time.

In order to achieve a high surface area LFP cathode with the indicatedrate capability using the metrics described above, the iron phosphateprecursor(s), dopant precursor(s), carbon source(s), and lithiumsource(s) may be thoroughly intermingled during the mixing, milling, anddrying processes without phase separation on the microscopic level.Thus, a key teaching contained herein allows one skilled in the art toproduce a dried powder containing the precursors described above thathas a specific thermal profile, as measured using thermal gravimetricanalysis (TGA) techniques, which, upon calcination, results in a finalLFP powder with the desired physical, morphological, and electrochemicalproperties. This characterization technique is monitoring the mass ofthe dried powder and quantifying the change in mass as a function of thesample temperature. The profile resulting from this analysis providesinsight into such things as decomposition patterns, degradationmechanisms, and reaction kinetics; all of which may occur at the optimaltemperature and at the optimal rate to achieve an LFP powder with thedesired characteristics.

As demonstrated by FIG. 17, there may be three key thermal zones thatare monitored. The first zone occurs between 75° C. and 125° C. with themaximum peak height occurring at approximately 100° C. or within a rangeof 95° C. to 105° C., in another example. The peak occurring in thisrange may have the second largest peak height when normalized using thepeak height from the signal collected in the second key thermal zone.The second zone may occur within a temperature ranging from 175° C. to250° C. with the maximum peak height occurring at approximately 225°C.±25° C. The third temperature zone may occur between 275° C. and 425°C. and may contain a bi-modal peak in which the peak recorded at a lowertemperature within the range may have a peak height that is moresuperior to that of the peak height associated with the peak recorded ata higher temperature within that same range in some cases. While twodistinct peak heights may be observed, it is not necessary for the peaksto be completely de-convoluted as illustrated in FIG. 17. No substantialsignal should be observed at temperatures above 500° C. in one example.It should also be noted that the rate at which the sample temperature isincreased should be 10° C./min while other heating rates (5-10° C./min)can still be acceptable to use. A faster rate may result in peak overlapwhich makes it harder to distinguish the individual thermal zones.

Deviation from the thermal profile described herein can be attributed tophenomena such as aggregates of non-homogenous particles that willresult in decomposition at different rates and temperatures, as afunction of heat and mass transfer. Dried powders with less fullyengaged ingredients will result in notably different decompositionkinetics which will result in different thermal profiles. It isimportant to note that a different thermal profile resulting from theconditions described above will not necessarily prevent the formation ofLFP, but one that does not meet the desired physical and morphologicalproperties as well as the needed electrochemical performance. Utilizingthis technique, therefore, can enable real time material screening, toensure more uniform mixing, milling, and spray drying processes.

The embodiments of the present disclosure focus on realizing an LFP withthe appropriate chemical, physical, structural, and electrochemicalproperties that allow for facile rate performance over a wide range oftemperatures while simultaneously achieving an FCC greater than 150mAh/g. The chemical formulation for the LFP described herein maycorrespond to Li_(z)Fe(_(1-x-y))V_(x)Co_(y)PO₄, where z≥1, 0.00≤x≤0.05,0.00≤y≤0.01 and where the ranges and optimal values to achieve thedesired result for x and y are described above. This system maydemonstrate maximum chemical stability and electrochemical performancewhen the molar ratio of total non-Li metals to phosphate ratioapproaches unity, where the total metals may be the sum of all thetransition metals incorporated into the LFP (for example, Fe+V+Co).Further, in some examples, other metals, or combination of metals, maybe used as a dopant. While unity is the optimal value for this metric, arange of 1.000 to 1.040:1 also results in the desired LFP properties andis utilized in the teachings herein. The main doping precursor used inthis present disclosure is VPO₄ which has a similar chemical structureto FePO₄, especially when considering the anions of the two species areidentical and therefore a higher doping efficiency when compared toother V doping precursors such as V₂O₅ and NH₄VO₃. It should be notedthat this teaching is not limited to V, but all transition metal dopantsconsidered. Another such example described herein is when co-doping theLFP with Co as the minority second dopant. In that system, it was alsodemonstrated that the phosphate based Co dopant may result in a moreeffective doping strategy when compared to other Co species such as theoxide form or cobalt oxalate. All of these ranges are acceptable and canresult in an LFP with the desired physical properties andelectrochemical performance.

When holistically utilizing the teaching of the present disclosure, inone example, the iron phosphate precursor as disclosed herein may: (1)result in zero NH₃ emission during LFP synthesis; (2) contain a Fepercentage by weight that approaches 37%, or ranges from 28 to 29.5% and36 to 37% in other examples; (3) a P/Fe approaching unity with higherphase purity in order to alleviate the need for addition Fe and/or Psources; (4) a higher conversion efficiency from FePO₄ to LFP; (5)result in a significantly reduced moisture uptake in an ambientenvironment as demonstrated in FIG. 9; and (6) result in lower slurryviscosity during milling (due to low SSA) and enabling higher solidscontent thereby increasing throughput and production rate.

Turning to FIG. 18, an electrode assembly is illustrated which mayinclude the disclosed LFP electrochemically active material. In astackable cell configuration, multiple cathodes and anodes may bearranged as parallel alternating layers. In the example illustrated inFIG. 18, a stackable cell electrode assembly 1800 is shown. Electrodeassembly 1800 is shown to include seven cathodes 1802 a-1802 g and sixanodes 1804 a-1804 f. In one example, the cathodes may comprise the LFPsynthesized from PP FePO₄ or an SP FePO₄ as described above. In anotherexample, the cathodes may comprise the LFP synthesized from the abovedescribed FePO₄. Adjacent cathodes and anodes are separated by separatorsheets 1806 to electrically insulate the adjacent electrodes whileproviding ionic communication between these electrodes. Each electrodemay include a conductive substrate (e.g. metal foil) and one or twoactive material layers supported by the conductive substrate. Eachnegative active material layer is paired with one positive activematerial layer. In the example presented in FIG. 18, outer cathodes 1802a and 1802 g include only one positive active material facing towardsthe center of assembly 1800. One having ordinary skill in the art wouldunderstand that any number of electrodes and pairing of electrodes maybe used. Conductive tabs, such as tabs 1808, 1810 may be used to provideelectronic communication between electrodes and conductive elements, forexample.

In FIGS. 19A and 19B, a wound cell example 1900 is illustrated in whichtwo electrodes are wound into a jelly roll and housed within acontainer. The container housing the negative electrode, the positiveelectrode, the non-aqueous electrolyte and the separator.

The LFP synthesized from the above described FePO₄ as discussed inrelation to Table III and method 1100 shows improved properties whenused as an electrochemically active material in a battery. Thus, anelectrochemical cell is disclosed, the electrochemical cell comprising apositive electrode with an active material layer comprising an LFPelectrochemically active material doped with vanadium and cobalt and hasa total non-lithium metal to phosphate molar ratio of 1.000-1.040:1, anegative electrode, an ionic electrolyte solution that supports themovement of ions back and forth between the positive and negativeelectrodes, and a porous separator.

The current disclosure improves upon prior LFP formulations by teachingkey attributes associated with the precursor materials that result inthe desired physical properties and electrochemical performance. Theseattributes are associated with the precursor crystal structure, chemicalmakeup (especially the anion), Fe content in the precursor and phasepurity which can be identified through diffraction techniques and theP/Fe molar ratio of the iron phosphate precursor. Additional teachingsare associated with how to manipulate the pore size and pore sizedistribution of the final LFP through precursor selection andprocessing. The enhanced crystalline pore network results in a lowermoisture uptake at both the powder and electrode level. The LFP taughtherein demonstrated less moisture uptake than the material proposed inU.S. patent application Ser. No. 14/641,172, for example. This may bedue at least in part to the manipulation of the micro- and nanostructureof the PP FePO₄-LFP with smaller pore size diameters, which, in turn,may hinder the water uptake. The final LFP powder may provide morerobust electrochemical performance and enhanced stability over time.

Various modifications of the present disclosure, in addition to thoseshown and described herein, will be apparent to those skilled in the artof the above description. Such modifications are also intended to fallwithin the scope of the appended claims.

It is appreciated that all reagents are obtainable by sources known inthe art unless otherwise specified.

Patents, publications, and applications mentioned in the specificationare indicative of the levels of those skilled in the art to which theinvention pertains. These patents, publications, and applications areincorporated herein by reference to the same extent as if eachindividual patent, publication, or application was specifically andindividually incorporated herein by reference.

The foregoing description is illustrative of particular embodiments ofthe invention, but is not meant to be a limitation upon the practicethereof.

The foregoing discussion should be understood as illustrative and shouldnot be considered limiting in any sense. While the inventions have beenparticularly shown and described with references to preferredembodiments thereof, it will be understood by those skilled in the artthat various changes in form and details may be made therein withoutdeparting from the spirit and scope of the inventions as defined by theclaims.

The corresponding structures, materials, acts and equivalents of allmeans or steps plus function elements in the claims below are intendedto include any structure, material or acts for performing the functionsin combination with other claimed elements as specifically claimed.

Finally, it will be understood that the articles, systems, and methodsdescribed hereinabove are embodiments of this disclosure—non-limitingexamples for which numerous variations and extensions are contemplatedas well. Accordingly, this disclosure includes all novel and non-obviouscombinations and sub-combinations of the articles, systems, and methodsdisclosed herein, as well as any and all equivalents thereof.

1-20. (canceled)
 21. A method to form lithium iron phosphateelectrochemically active material for use in an electrode in anelectrochemical energy storage device, comprising: (a) mixing a vanadiumdopant source, a lithium source, a carbon source, an iron phosphatesource with an iron content of at least 28 wt. % and a phosphate to ironmolar ratio of 1.000-1.040:1, and optionally a co-dopant in a solvent toform a slurry; (b) milling the slurry; (c) drying the milled slurry toform a lithium iron phosphate precursor powder; and (d) firing the driedmilled slurry to obtain the lithium iron phosphate electrochemicallyactive material, wherein the lithium iron phosphate electrochemicallyactive material comprises the vanadium dopant and/or co-dopant partiallysubstituting the Fe in a crystal lattice structure, a phosphate to ironmolar ratio of 1.000-1.050:1, and a total non-lithium metal to phosphatemolar ratio of 1.000-1.040:1.
 22. The method of claim 21, wherein thelithium iron phosphate electrochemically active material has a surfacearea greater than about 25 m²/g, a tap density within a range of 1.0-1.4g/mL, FCC greater than 150 mAh/g, and a 10 C discharge capacity greaterthan 140 mAh/g.
 23. The method of claim 21, wherein the vanadium dopantis contributed by an oxyanion precursor species such as an oxide,carbonate, oxalate, phosphate or other such source for which thevanadium is considered the cation.
 24. The method of claim 23, whereinthe vanadium dopant source is vanadium phosphate (VPO₄), ammoniummetavanadate (NH₄VO₃), or a combination thereof.
 25. The method of claim21, wherein the co-dopant comprises cobalt.
 26. The method of claim 25,wherein the cobalt co-dopant is cobalt oxalate (CoC₂O₄), ammonium cobaltphosphate (NH₄CoPO₄), or a combination thereof.
 27. The method of claim21, wherein an ammonia emission source is confined to the vanadiumdopant, a co-dopant, or a combination thereof.
 28. The method of claim27, wherein the ammonia emission is substantially zero.
 29. The methodof claim 21, wherein the iron phosphate source comprises a hexagonalcrystal structure.
 30. The method of claim 21, wherein the lithium ironphosphate precursor powder comprises a differential thermal gravimetricloss profile with observable peaks within three temperature ranges, 75to 125° C., 75-250° C., and 275-425° C.
 31. The method of claim 30,wherein the 275-425° C. temperature range contains a bi-modal peak inwhich the peak recorded at a lower temperature within the rangecomprises a peak height that is more superior to the peak heightassociated with a peak recorded at a higher temperature within that samerange, and wherein there is no substantial peak above 500° C.
 32. Themethod of claim 31, wherein a total thermal mass loss from the lithiumiron phosphate precursor powder is less than 40% when heated fromapproximately 25° C. to 600° C.
 33. The method of claim 21, wherein thevanadium dopant is present at 2 to 4 Mol. % and the cobalt co-dopant ispresent at 0.0 to 0.5 Mol. %.
 34. The method of claim 21, wherein themethod results in a DCR of less than 9 ohm when measured for a 20 mAhdouble layer pouch cell at −20° C. 35-37. (canceled)
 38. A method forsynthesizing vanadium phosphate comprising: pre-mixing a vanadiumprecursor and a phosphate precursor in a solvent to form a slurry;stirring the slurry at an elevated temperature; adding a carbon sourceor reducing agent to the slurry; milling the slurry; spray-drying themilled slurry into a powder; and reducing the vanadium precursor to atrivalent vanadium species using a temperature programmed reaction underan inert atmosphere.
 39. The method of claim 38, wherein the inertatmosphere is comprised of nitrogen, hydrogen, a noble gas, or acombination thereof.
 40. The method of claim 38, wherein the solventcomprises an organic solvent, an alcohol, water, or a combinationthereof.
 41. The method of claim 38, wherein the vanadium precursorcomprises vanadium in the plus five oxidation state.
 42. The method ofclaim 41, wherein the vanadium precursor is an oxide or vanadatespecies.
 43. The method of claim 38, wherein the vanadium to phosphatemolar ratio is between 0.9-1:1. 44-46. (canceled)